There are proposed the following techniques in the configuration of a signal input circuit for a semiconductor integrated circuit.
For example, FIG. 1A shows the configuration of a general signal input circuit. An input circuit 10 receives differential signals Inp and Inn, and outputs an output signal Vout corresponding to difference information Vin between the differential signals.
FIG. 1B shows a circuit diagram of a signal input circuit in a differential configuration. The input signals Inp and Inn are input to the gate terminals of input transistors MN1p and MN1n, respectively. A current that is generated by a constant current generation transistor MNc and a bias voltage Vbias is distributed to two according to a difference voltage between the input signals, and flow in two load resistors RL1p and RL1n. Output signals Outp and Outn are output according to a product of load resistances RL1p and RL1n and currents that flow in the load resistances RL1p and RL1n, respectively.
When the differential signal Vin of the input signal is 0, it is general to design the circuit so that the differential signal Vout of the output signal becomes also 0. In order to achieve this, the input transistors MN1p and MN1n are so designed as to have the same characteristic, and the load resistances RL1p and RL1n are also so designed as to have the same characteristic.
However, when the characteristics of the input transistors MN1p and MN1n are varied for any reason, there occurs a phenomenon that the differential signal Vout does not become 0 even if 0 is supplied to the differential signal Vin. The same is applied to the variation of the load resistances RL1p and RL1n. 
An error in the differential signal which is caused by the above circuit characteristic can be represented by an equivalent circuit shown in FIG. 1C. That is, an error voltage Vos that offsets the input signal is applied to any one of the input signals. The error voltage Vos takes a positive or negative value according to the variation of the circuit characteristic. The error voltage Vos is generally called “offset voltage”.
According to the study of the present inventors, as a method of correcting the circuit characteristic variation of the signal input circuit described above, there are proposed the following techniques.
For example, FIG. 2A shows a first conventional art for correcting the circuit characteristic variation of the signal input circuit. The first conventional art is configured in such a manner that an output signal of an input circuit 20 is detected by the aid of an observation signal 23 outside of a semiconductor integrated circuit, and a circuit characteristic adjustment signal 22 corresponding to the observation signal is supplied to the semiconductor integrated circuit. The input circuit 20 has a function of adjusting the characteristic of the input circuit per se upon receiving the adjustment signal 22.
Also, for example, FIG. 3A shows a second conventional art for correcting the circuit characteristic variation of the signal input circuit. The second conventional art is configured in such a manner that there is provided an amplifier circuit 32 (automatic zero amplifier) which receives a signal from an input terminal of the input circuit 30, and the circuit characteristic variation of the input circuit 30 is corrected by the aid of an output signal of the automatic zero amplifier 32. The automatic zero amplifier 32 has a function of adjusting its own circuit characteristic variation.